1. Field of the Invention
The invention is directed to an electrical penetration assembly for effecting a vapor and pressure seal for a conductor passing through the two freestanding walls of a nuclear reactor containment that are subject to relative movement due to a seismic event or thermal expansion of the walls and the conductor.
2. Background of the Invention
In most types of nuclear reactors, the reactor core is contained in a reactor vessel constructed of steel. The reactor vessel is housed in a containment building, or vessel, which is designed as a pressure vessel constructed from walls of concrete and steel. The containment vessel provides biological shielding and is pressure sealed to prevent the escape of vapors, liquids and other reactor materials in the event of leakage in the primary system. For the operation of the reactor, it is necessary to pass a variety of electrical conductors including power lines and control signal lines through the walls of the containment vessel.
In one type of containment vessel, two freestanding walls, one of concrete and one of steel are used to surround the nuclear reactor. The inside wall is normally referred to as the containment wall and the outside wall is normally referred to as the shield wall. The containment and shield walls are generally separated by an annulus of varying width. Conductors that must pass through the containment vessel are fixed to the walls of the containment vessel in penetration assemblies which are mounted in penetration nozzles built into the walls of the containment vessel during their initial fabrication. The freestanding walls of the containment vessel will move independently when subjected to motion due to a seismic event or due to a temperature variation between the walls or the walls and the conductor. This motion can cause damage to the conductor or the penetration assembly and this could allow the escape of vapors and other reactor materials in the event of a primary system leak.
Another problem encountered when installing electrical penetrations in a containment vessel having two spaced freestanding walls in that penetration nozzles built into each wall may not be in exact alignment. With low power conductors or signal conductors, this is not a serious problem due to the flexibility of the conductors. However, with medium and high power conductors, which are relatively stiff, misalignment of the penetration nozzles will cause an undesirable loading of the conductors or their protective covering when they are forced into place in a pair of misaligned penetration nozzles.
A variety of electrical penetration assemblies have been proposed and used. Among their deficiencies they are relatively expensive and complicated, they require undesirable maintenance during the life of the reactor plant, and they require an undue amount of costly on-site assembly and installation time. Some prior art electrical penetration assemblies have included provisions for minimizing damage to the electrical penetration assembly due to the relatively slow and gradual type of movement associated with thermal expansion of the conductors. However, these prior art electrical penetration assemblies have all been designed to pass through a single wall and the prior art does not present a solution to the problem of how to compensate for the relatively quick and violent relative movements of two freestanding walls due to a seismic event.
It is therefore the principal object of the invention to provide an electrical penetration assembly for two freestanding walls which prevents damage to either the conductor or the penetration assembly during a seismic event.
It is another object of the invention to provide an electrical penetration assembly which prevents damage to either the conductor or the penetration assembly due to thermally induced stresses resulting from the expansion of the conductor or the relative movement of the walls.
It is another object of the invention to provide an electrical penetration assembly for two freestanding walls which minimizes installation difficulties resulting from unavoidable misalignments of the walls or the penetration nozzles mounted therein.
It is another object of the invention to provide an electrical penetration assembly with a simplified modular construction.
It is another object of the invention to provide an electrical penetration assembly having two pressure barriers in series.
It is another object of the invention to provide an electrical penetration assembly which maintains sealing integrity throughout the plant lifetime with a minimum of maintenance.